1. Field of the Invention
The present invention relates to optical systems and more particularly to a method and apparatus for aligning the optical axis of mirrors. The invention further relates to optical alignment of off axis a spherical or non-circularly symmetric mirrors.
2. Related Technical Art
High quality reflective mirrors are used in a variety of advanced optical applications such as image scanners or sensors, image duplication, or image digitizing systems. In high resolution optical systems the mirrors must possess high quality optical surfaces and be very accurately aligned to provide a desired high resolution or high image registration. Therefore, it is necessary to provide methods of manufacturing mirrors with very precisely defined focal patterns and that are capable of high accuracy alignment within a given optical system. The alignment process may include system alignment, but also includes initial alignment of the mirror within a mounting structure for later use.
When flat, circular, or circularly symmetric mirrors are employed it is often simple to align the mirrors by simply using a mechanical measurement of the focal point of the mirror. The size and circular curvature of such mirrors are easy to measure, and from this the focal point or axis is also easily determined. Light beams can also be reflected from the mirror surface and observed to see where a common intersection point of reflected light is or to trace predetermined patterns. Another method is to rotate the mirror while reflecting a collimated light beam or an image from its surface and adjust the mirror's axis of rotation until the reflected image is stationary. To achieve very high accuracy alignment requirements, a variety of interferometric techniques can also be used to detect mis-alignment using incident and reflected light beams and images. However, when dealing with a spherical or parabolid shapes or more complex geometries and offset mirrors, the traditional interferometric methods do not work well.
For mirrors in which there is no circular central portion, the physical structure of the mirror prevents very accurate determination of the focal axis or makes this axis very hard to locate. In this arrangement there is no central portion from which to easily reflect a beam of light or an image. One method of alignment is to assume that any zonal error is concentric or symmetric about the central optical axis if alignment is perfect. An interferogram is used to detect or focus to a mirror zone, which is then used to establish the optical axis. Likewise, Ronchi tests can be used to find a pattern passing a mirror zone. Several tests can be performed to observe several portions of the mirror surface and then be compiled to establish a single, composite, optical axis. However, these methods have been found to be in error by as much as twenty percent.
Unfortunately, off set mirrors and mirrors having complex surface shapes are needed for highly advanced optical systems. Therefore, what is needed is a method and apparatus for providing highly accurate alignment of such mirrors. At the same time, a new mirror alignment technique is needed that provides very accurate measurement of mirror alignment for a variety of mirror configurations. It is also desirable for any alignment technique to be usable with a minimum of expense and complication.